In the field of communication systems, a current objective is to provide ever-greater data bandwidths without requiring additional cabling or optical fiber lines. To this end, lightwave communication systems now use wavelength division multiplexing (WDM) in which multiple lightwave communication channels are transmitted over the same optical fiber by using a number of different wavelength channels. More recently, the spacing between these different wavelength channels has decreased, leading to dense WDM (DWDM) systems. The DWDM systems require a different laser wavelength for each wavelength channel. Additionally, a wavelength dispersive element such as an arrayed waveguide grating (AWG) is required to separate the different wavelength channels and to direct the resulting individual wavelength channels to different photodetectors.
The DWDM systems place very stringent requirements on a number of system components. The wavelength of each laser must be very carefully controlled, as a drifting laser wavelength will lead to crosstalk with adjacent wavelength channels. As the power of each laser is relatively limited and the optical fiber used in transmission is lossy, long fiber optic links require the added complexity of optical amplifiers at appropriate repeater intervals. Because the spacing between wavelength channels has decreased, the AWG must very accurately separate the different wavelength channels or crosstalk will likely occur. The strict tolerances on the AWG are due in part to the use of a single photodetector design for all wavelength channels. That is, the photodetector is responsive to light from each of the wavelength channels and cannot differentiate between the various wavelength channels.